The present invention relates to a process and reactor design for the polymerization of monomers in a nonaqueous system. More particularly the present invention relates to a reactor which is operated partially full, that is, a portion of the volume of the reactor is vapor space. The vapor space additionally contains an agitator which aids in reducing volatile monomer accumulating in the vapor space under certain reaction conditions. The reactor is particularly suited for use in an anionic polymerization process.
There are many known examples of solution polymerizations involving one or more volatile monomeric components. Often it is desired to control the degree of polymerization of one component of a reaction mixture. For example, in the polymerization of block copolymers containing one component that is relatively volatile, such as butadiene, it may be desired at some stage to polymerize a relatively pure block of such volatile monomer, and thereafter to continue the polymerization by adding a second monomer to the reactor to form a relatively pure second polymer block. In reactors containing a vapor space, it has been discovered that significant quantities of the volatile monomer may remain in the vapor space of the reactor after polymerization of the dissolved portion of the monomer is completed. Unless the quantity of monomer held in the vapor space is reduced, this remnant may subsequently reenter the reaction mixture and polymerize, thereby destroying the purity of later formed polymer blocks. This problem of residual volatile contamination may also occur in subsequent batch polymerizations utilizing the same reactor unless the reactor is thoroughly purged. In addition to the above problem of polymer contamination, the loss of quantities of volatile monomer into the vapor space of the reactor may result in an undesirable molecular weight variation from desired polymer composition and monomer contamination of the resulting polymer product.
Although a condenser may be employed to remove volatile components from the vapor space of a reactor, often even this technique is not adequate to completely remove the desired amount of volatile monomer. Thus, it would be desirable if there were provided a reactor having a vapor space suitable for use in the solution polymerization of volatile monomers that provides a means to reduce the concentration of such volatile monomers in the vapor space.
It would further be desirable if there were provided an anionic polymerization process that is characterized by a means for reducing the amount of volatile monomer that partitions into the vapor space.